This invention relates to the detection of light from a subject to obtain data suitable for the formation of an image of the subject and, more particularly, to the scanning of light past groups of photodetectors arranged in rows transverse to a path of the scanning, and wherein each group of photodetectors is responsive to a separate portion of a spectrum of the light.
The term "light" as used herein includes the visible spectrum as well as invisible spectra such as ultraviolet light and infrared light. The photodetectors employed in the selection of the light are fabricated generally of semiconductor materials responsive to radiation of a specific part of the electromagnetic spectrum such as infrared detectors or detectors of visible light. It is common practice today to construct an array of photodetectors upon a common substrate of semiconductor material.
An array of photodetectors is particularly useful in the scanning of light emitted by or reflected from a subject to obtain data of the subject which can be used in the construction of an image of the subject. By scanning the light of the subject past a set of photodetectors, each of the photodetectors provides a signal in response to the incident light. Preferably, the scanning is accomplished along a path perpendicular to a line array of the photodetectors. For each scan line, the amplitudes of the signals as a function of time represents one line of pixels (picture elements) in an image of the subject.
Typically, the output signal of each photodetector is connected to electronic circuitry which samples the output signal at a rate synchronized with a rate of the light scanning so as to provide a predetermined number of signal samples for each line scanned. Signal processing circuitry extracts the various signal samples from the photodetectors, and arranges the samples to form an image of the subject. Various forms of filtering may be employed on each signal sample, or on a succession of the samples to reduce the influence of noise so as to enhance the image.
A problem arises in that detector elements have a responsivity to light which varies with the frequency of the light. The effects of the varying responsivity are compounded by the fact that, in the case of illumination of the subject by an external source of light, the source of light may have an intensity which also varies as a function of frequency. As a result, these systems are limited in their capacity to produce accurately a multicolored image of the subject.
For example, one method of producing a multicolored image is to employ red, green and blue filters in front of the array of photodetectors to obtain images in the primary colors of red, green, and blue. The three images are combined to produce the desired multicolor image. However, in view of the foregoing variation in illuminating light and in detector responsivity as a function of light frequency, the relative intensities of the images in the primary colors may be in error. An attempted correction of the situation to produce uniformity by attenuating the signal of the color image having an excessive intensity simply worsens the signal-to-noise ratio with a resultant degradation of the multicolor image.